In some wine varieties, such as muscats, the aroma compounds exist in two forms, free and bound. The free fraction consists of odoriferous volatile substances, chiefly terpenols. The bound fraction contains precursors of terpenols, especially non-odoriferous diglycosides, formed from .alpha.-L-rhamnopyranosyl-.beta.-D-glucopyranosides (designated Rha-Glc), from .alpha.-L-arabinofuranosyl-.beta.-D-glucopyranosides (designated Ara-Glc) and from .beta.-D-apiofuranosyl-.beta.-D-glucopyranosides (designated Api-Glc), in which the glucopyranose provides the link between the terpene residue (designated Terp) and the disaccharide, according to the formulae: EQU Rha(1.fwdarw.6)Glc-Terp EQU Ara(1.fwdarw.6)Glc-Terp EQU Api(1.fwdarw.6)Glc-Terp
The aroma fraction in the form of precursors is most often much larger than the free aroma fraction (typically by a factor of 3 to 10), and it can reach high concentrations, of the order of a few milligrams per liter.
Taking into account, in addition, the particularly low threshold of olfactory perception and the aromatic quality of terpene alcohols, there is, in these vine varieties, a most important unexploited aroma potential.
The terpene glycosides, present in the juice, may be hydrolysed using commercial enzyme preparations with a wide variety of specifications. The enzymatic liberation of terpenols, which reflects the free natural aroma of the fruit more faithfully than that revealed by thermal hydrolysis at the pH of the juice, is hence possible; however, the control of this liberation for an industrial exploitation of the aroma potential presupposes that the glycosidases responsible for the hydrolyses are defined and their mechanism of action established.